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SMT processing sampling inspection method classification
PCBA Tech
SMT processing sampling inspection method classification

SMT processing sampling inspection method classification

2021-11-06
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Author:Downs

The following is mainly an introduction to the classification of sampling inspection methods for SMT chip processing:

      At present, many sampling inspection programs and systems with different characteristics have been formed, which can be roughly classified according to the following aspects.

      1. Classification according to the characteristics of production SMT quality indicators

      The characteristic quantity that measures the quality of the product is called the quality index of the product. Quality indicators can be divided into two categories: measurement indicators and counting indicators according to their measurement characteristics. Measurement indicators refer to quantitative data indicators such as the purity of the material, the size of the processed part, the chemical composition of the steel, and the life of the product. The counting index can be divided into two types: piece counting index and counting index. The former is measured by the number of unqualified products, and the latter refers to the number of defects in the product, such as the number of appearance defects on a square meter of cloth, and the number of defects on a casting. The number of bubbles and trachoma, etc.

      According to the quality index classification, the sampling inspection methods of SMT product quality inspection are also divided into two types: sampling inspection and measurement sampling methods.

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      1. The measurement sampling method is to take a certain number of samples (samples) from a batch of products, inspect the quality of each sample in the sample, and then compare with the specified standard value or technical requirements to determine whether the batch of products is qualified Methods.

      2. The counting sampling method is to take a certain number of samples (samples) from the batch products, inspect the quality of each sample in the sample, determine whether it is qualified or unqualified, and then count the number of qualified products, and the specified "qualified judgment number" Comparison, the method to determine whether the batch of products is qualified. 

      Sometimes, the sampling inspection method by attributes and the sampling inspection method by quantity can also be mixed. If a certain quality parameter or a few quality parameters of the product are selected for measurement sampling, the counting sampling method is implemented for most of the remaining quality parameters to reduce the calculation workload and obtain the required quality information.  

      Second, according to the type of sampling method classification

      Sampling inspection methods can be divided into two categories: adjustment type and non-adjustment type.

      The rescue type is to contact the transfer end party with several different sampling plans to create a complete sampling inspection system. Then, according to the quality of each batch of products, the sampling inspection plan is adjusted according to the transfer end party, which is normal, strict or Relaxing the conversion of random inspection schemes. ISO2859, ISO3951 and GB2828 standards are all examples of this. The rescue-type random inspection methods are used for quality inspections of continuous batches of products with consistent quality.

      The non-rescue single-sampling search plan does not consider the quality history of the product batch, and there is no transfer of the end party in the operation. Therefore, it is easy to control by the quality inspection staff, but it is only suitable for the quality inspection of the Guao batch.

      Regardless of the sampling method, they have the following three characteristics in common:

      1. Products must appear in the form of "inspection batches" ("batch" for short). Inspection batches are divided into continuous batches and isolated batches. Continuous batches refer to batches that have a close relationship between product quality or continuous production and are continuously submitted for acceptance. like:

      ① There is no change in product design, structure, process, and materials;

      ②There is no change in the manufacturing site;

      ③The intermediate production stop time shall not exceed one month.

      A single submitted inspection batch or a batch to be picked up can not use the quality information provided by the most recently inspected batch, which is called an isolated batch.

      2. Qualified batch does not mean that every product in the batch is qualified, and unqualified batch does not mean that every product in the batch is unqualified. Sampling inspection only guarantees the overall quality of the product, not the quality of each product. That is to say, in the sampling inspection, there may be two kinds of "errors" or "risks. One is the error of misjudging qualified batches as unqualified batches, also known as "producer risk", often denoted as α, generally α The value is controlled at 1%, 5% or 10%. The other is to misjudge the unqualified batch as a qualified batch, also known as "user risk", often recorded as β, generally β is controlled at 5%, 10 %.

      3. The non-conforming product rate of the SMT sample is not equal to the non-conforming rate of the submitted batch.

The sample is randomly selected from the batch submitted for inspection. The so-called random extraction means that every time it is drawn, all unit products in the batch are equally likely to be drawn and are not subject to anyone's will. The sample extraction time can be in the batch formation process or after the batch formation. The random sampling number can be checked according to the random number table, or can be determined according to standards such as GB10111.

      3. Classification according to the number of sampling inspections

      According to the number of sampling inspections, it can be divided into one, two, multiple and sequential sampling inspection methods.

      1. One-time sampling method, the method is the simplest, it only requires sampling a sample to make a judgment on whether a batch of products are qualified.

      2. The second sampling method is to take the first sample for inspection, and if it can judge whether the batch of products is qualified or not, the inspection will be terminated. If the judgment cannot be made, a second sample shall be drawn, and then re-inspected and judged whether it is qualified or not.

      3. Multiple sampling method "The principle is the same as that of the second sampling method. The sample size of each sampling is the same, that is, n1=n2=n3...=n7, but the number of sampling inspections is large, and the number of qualified judgments and the number of unqualified judgments are also large. The ISO2859 standard provides 7 sampling inspection programs. However, my country's GB2828 and GB2829 have implemented 5 random inspection programs.

      4. The sequential sampling method is equivalent to the limit of multiple sampling methods. Only one unit of SMT product is randomly selected for inspection at a time. When the batch can be judged as qualified or unqualified, the inspection shall be terminated.